Triarylamines—A Versatile Molecular Building Block for Optoelectronic Materials

Triphenylamine (TPA), a propeller-shaped, electron-rich molecular motif has been an integral part of several functional materials that require π–conjugation, an extension of molecular structure with the ease of functionalization, and judiciously tailorable structures for the desired applications. The library of molecules developed based on TPA as a core or terminal group is numerous and predominantly projected towards the choice of materials for optoelectronic applications including light-harvesting molecules for solar cells, charge transport materials for optoelectronic devices, light emitters for organic light emitting devices, luminescent probes to monitor the biomarkers and environmentally toxic species. The envisaged applications for the TPA-based molecules rest on tuning their optoelectronic properties which are generally achieved by introducing several functional moieties. The electronic energy levels, highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), are highly sensitive to the number of functional groups substituted in the TPA core, substitution position, and nature of the functional group thereby providing the enormous opportunity to create a molecular library with the structurally diverse functionalities. This review addresses the structure–property relationship of molecules having TPA as an important building block of small molecules, oligomers, and polymers. To define generally, TPA is regarded as a donor (D) and those groups which are relatively electron-deficient are denoted as acceptors (A). The simplest molecular configuration is DA, denoted as dipolar molecules which have been further build-up with multiple acceptor groups to form DA2 and DA3, respectively, denoted as quadrupolar and octupolar molecules. Being substituted with electronically unequal groups connected through π-conjugation, intramolecular charge transfer from donor to acceptor occurs upon photoexcitation, and in fact, it is an important phenomenon that drives the potential of these molecules for several applications. Understanding the intramolecular charge transfer process is essential to elucidate the structure–function relationship and has been summarized in this review comprehensively. The underlying phenomena in constructing the sensors using these chromophores, aggregation-induced emissive luminophores, and construction of framework materials such as metal–organic framework and covalent organic framework are also highlighted.